JPH09210918A - Optical system for detecting foreign matter on silicon wafer surface - Google Patents

Abstract

(57) Abstract: Noises due to crystal defects and an oxide layer existing on the surface or inside of a silicon wafer are reduced, and a foreign substance of 0.1 μm or less existing on the surface is detected. An argon laser oscillator tube 511 that outputs a P-polarized laser beam L _T having a wavelength of 488 nm, and a focusing lens 512 that focuses this and projects it onto the surface of the wafer 1 at a Brewster angle θ _B. The light projecting system 51 and the light receiving angle θ _R whose optical axis is equal to the Brewster angle θ _B are set,
Light receiving system 52 including a condenser lens 521 having a condenser angle of about 20 °, an S polarization filter 522, and a photomultiplier tube 523.
It is composed of

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system for detecting foreign substances existing on the surface of a silicon wafer.

[0002]

2. Description of the Related Art Silicon wafers, which are the materials for semiconductor ICs,
A single crystal silicon ingot is made from high-purity polycrystalline silicon by a pulling method, etc., sliced into a thin plate, the surface of which is polished to a mirror finish, and foreign matter adhering to the surface is washed. Used.
Despite this cleaning, some foreign matter may remain on the surface, and if it remains, it impairs the quality of the IC. Therefore, the foreign matter inspection apparatus inspects for the presence of foreign matter, and if there is, further cleaning is repeated. Which measures are being taken.

An outline of a conventional foreign matter inspection apparatus will be described with reference to FIG. (a) shows the basic structure of the foreign matter inspection device,
It includes a moving table 2, an inspection optical system 3, a data processing unit 4, and the like. A silicon wafer (hereinafter simply referred to as a wafer) 1 to be inspected is placed on a rotary / movable table 2, and a laser beam L _T output from a laser light source 31 of an inspection optical system 3 is collimated by a collimator lens 32. After passing through the mirror 33, the light is focused on the spot S _P by the focusing lens 34 and projected vertically onto the surface of the wafer 1. By the rotation and movement of the rotation / movement table 2 and the movement in the radial direction, the spot S _P spirally scans the surface of the wafer 1 and the entire surface of the wafer 1 is scanned. When the foreign matter e exists on the surface, as shown in (b), the spot S _P scatters the scattered light S _e in a random direction by the foreign matter e, and a part of the scattered light S _e forms a condensing lens 35 having an optical axis of 45 °. Collected by the photomultiplier tube (PMT)
The light is received by 36, and its output signal is input to the foreign matter detection circuit 41 of the data processing unit 4, compared with an appropriate threshold value v set therein to remove noise, and the foreign matter e and its size are detected. . The detected size data is input to the data editing circuit 42, the XY coordinate values of the foreign matter e are added to edit the foreign matter data, and the printer 43 prints and displays the map.

In the inspection optical system 3 described above, an argon laser tube that oscillates a laser having a short wavelength is used as the laser light source 31, the diameter of the spot S _P is set to a minute few μm to increase the intensity, and a condenser lens is used. Since a large condensing angle is used for 35 to effectively condense the scattered light S _e and the PMT 36 has a large amplification factor, the foreign matter e can be detected up to a diameter of about 0.15 μm. .

Recently, the integration density of IC memory is 16
It has been improved from megabits to 64 megabits or more, and foreign matter management of IC memory of 16 megabits or less was said to be harmful if it had a diameter of 0.2 μm or more, but 0.1 μm for 64 megabits or more. The following foreign substances are also considered to impair the quality. On the contrary,
The intensity of the spot S _P , the converging angle of the condenser lens 35,
Even if the amplification factor of the PMT 36 is further increased, noise is increased accordingly, so that it is difficult to detect the foreign matter e of 0.1 μm or less. There are various causes of this noise. Among them, there are crystal defects (COP: crystal Originated Part) which are generated in the manufacturing process of the wafer 1 and exist on the surface or inside.
icles) and oxide layers (OSF; Oxidation Induced Stacki)
ng Folt), and the scattered light scattered by these becomes noise, and the S / N ratio of the scattered light S _e of the foreign matter deteriorates.

FIG. 7 is a sectional view showing a model of COP and OSF. The COP is dish-shaped on the surface of the wafer 1 and is shallow,
Or it exists rather deeply, or it exists inside. OS
F is also present on the surface and inside, and scatters the laser beam L _T like foreign matter e.

[0007]

In order to improve the detection performance of the foreign matter e more than before, noise due to COP and OSF is reduced to improve the S / N ratio of the scattered light S _e of the foreign matter e. It is necessary. The present invention improves the inspection optical system 3 described above to reduce noise due to COP and OSF,
An object is to detect a foreign matter having a size of 0.1 μm or less.

[0008]

SUMMARY OF THE INVENTION The present invention is an optical system for detecting foreign matter on the surface of a silicon wafer, which solves the above-mentioned problems, wherein a P-polarized laser from a laser light source is applied to the surface of the silicon wafer having a refractive index n. Beam the Brewster angle θ _B
And a projection system that minimizes the P-polarized specular reflection light from the surface of the projected laser beam to a minimum value close to zero, and is provided symmetrically with respect to the projection system, and the optical axis is the Brewster angle θ. _B
Is set to an acceptance angle θ _R equal to
A condensing lens for condensing randomly polarized scattered light due to a foreign substance existing on the surface, an S polarization filter for extracting the S polarized component of the condensed scattered light, and an extracted S polarized component for receiving the foreign substance And a light receiving system including a light receiver that outputs a signal.

The above-mentioned light source of the projection system has a wavelength λ of 4
An argon laser tube that outputs a P-polarized laser beam of 88 nm is used, the refractive index n of the silicon wafer for this wavelength λ is 4.0, and the Brewster angle θ _B for this refractive index n is 76 °. θ _R to about 76 °,
The converging angle of the condensing lens is set to about 20 °.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS First, the Brewster angle θ _B which is important in the present invention will be described with reference to FIGS. 1 and 2. In FIG. 1, from the air side with a refractive index n ₁ = 1
When the light flux L of the [P] polarized wave and the [S] polarized wave are projected onto the surface of the dielectric having the refractive index n ₂ at appropriate incident angles θ ₁ , the light flux L is specularly reflected by the surface, and It refracts at a refraction angle θ ₂ determined by the refraction index n _2, passes through the surface, and enters the inside of the dielectric. In this case, the regular reflection light R [P] of the [P] polarized wave and the transmitted light T [P], and the regular reflection light R [S] of the [S] polarized wave and the transmitted light T [S] are incident angles. θ ₁
It changes greatly depending on the situation. Now, let r be the energy reflectance of the surface and t be the energy transmittance of the surface, then the reflectance r [P] for the [P] polarized wave, the transmittance t [P], and the reflectance r for the [S] polarized wave. [S] and transmittance t [S] are given by the following known equations. r [P] = [tan (θ ₁ −θ ₂ ) / tan (θ ₁ + θ ₂ )] ² ……… (1) t [P] = 1−r [P] ……… (2) r [S ] = [Sin (θ ₁ −θ ₂ ) / sin (θ ₁ + θ ₂ )] ² ……… (3) t [S] = 1−r [S] ……… (4) where θ ₂ is a snell Is determined by the following equation. n ₁ sin θ ₁ = n ₂ sin θ ₂ (5) In equation (1), when θ ₁ + θ ₂ = π / 2, tan
Since (θ ₁ + θ ₂ ) becomes infinite, r [P] becomes 0 and the specularly reflected light R [P] disappears, and θ _{1 at} this time is the Brewster angle θ _B.

In the present invention, it is necessary to know the Brewster's angle θ _B of the silicon wafer 1, which can be calculated from the refractive index n of silicon. However, the refractive index n of silicon or glass generally changes depending on the wavelength λ, and for example, 3.43 is published as the refractive index n of silicon for a wavelength of 3 μm (Optical Engineering Handbook: p307. reference). However, since the wavelength λ of the laser beam used in the detection optical system varies depending on the type of laser light source, an accurate refractive index n for that wavelength λ is desired. On the other hand, the inventor of the present invention
The Brewster angle θ _B of the silicon wafer 1 is directly obtained by an experiment.

FIG. 2 is a curve diagram showing the above experimental data. The horizontal axis shows the incident angle θ ₁ , and the vertical axis shows the reflectance r and the transmittance t in%. An argon laser tube is used as a laser light source, the angle of a linearly polarized wave having a wavelength of 488 nm oscillated by the argon laser tube is changed to generate a [P] polarized wave and an [S] polarized wave, which are projected on the surface of the silicon wafer 1, The reflectance r [P], the transmittance t [P], and the reflectance r for the incident angle θ ₁
[S] and transmittance t [S] are measured, and the actual measurement values shown by x in the figure are obtained. The reflectance r [P] is θ ₁ = 7
Since it shows a minimum value at 6 °, it is tentatively defined as the Brewster angle θ _B, and therefore the refraction angle θ ₂ is set to 14 °.

Here, just in case, this Brewster angle θ
Consider whether _B is correct. In equation (5) above, θ
Substituting ₁ = 76 ° and θ ₂ = 14 ° to obtain n ₂ ,
The refractive index n = 4.0 of the silicon wafer is obtained. Equation (5)
Θ ₂ for various θ ₁ is obtained by setting n ₂ of 4.0 to
Inserting these into equations (1) to (4), r [P], t [P],
When r [S] and t [S] were calculated, the calculated values shown in the figure were obtained. The measured values of r [P] and t [P] have a difference of about 1% in the vicinity of the Brewster angle θ _B (76 °), but for the other measurement ranges, they are almost exactly the calculated values. As a result, it is confirmed that the measured value is correct. However, the measured values of r [S] and t [S] do not match the calculated values, and the reason is not clear, but for the time being, it does not matter.

FIG. 3 shows the structure of the foreign matter detecting optical system 5 of the present invention constructed on the basis of the above experimental results. Projection system
Reference numeral 51 designates an argon laser oscillator tube 511 as a laser light source, and a laser beam L _T having a wavelength of 488 nm oscillated by the laser beam L _T as a P-polarized wave, which is focused by a focusing lens 512 and a Brewster angle θ _{B with} respect to the surface of the wafer 1. Project at (76 °). The intensity of the regular reflection light R [P] by the surface is about 1% of the projected laser beam L _T. On the other hand, the intensity of the transmitted light T [P] refracted on the surface has a transmittance t [P] of up to 9
Since it is 9%, most of it penetrates the surface and penetrates inside. The attenuation rate α of the wafer 1 is 1.28 × 10 ⁴ / cm
The transmitted light T [P] penetrating inside has a depth of, for example, 27% at 1 μm, 7.7% at 2 μm, and 3 μm.
2% and so on.

In the light receiving system 52, the optical axis is the Brewster angle θ _B.
And the S-polarizing filter 522, which has a light-receiving angle θ _R equal to, and has a light-collecting angle of 20 °, and the PMT.
It consists of 523. The above-mentioned specular reflection light R [P] of about 1% is
The light is collected by the condenser lens 521 of the light receiving system 52, but is not received by the PMT 523 because it is removed by the S polarization filter 522, and the projected laser beam L _T does not include [S] polarized waves. Therefore, the regular reflection light does not exist, and P
Of course, these do not become noise because they are not incident on MT523. On the other hand, the foreign matter e present on the surface is
Randomly polarized scattered light S _{e in} which both [P] and [S] components are mixed is scattered in a random direction, and a part of the scattered light is condensed by the condenser lens 521 and the [P] component by the S polarization filter 522. Are removed and the [S] component is received by the PMT 523.

According to FIG. 4, scattered light S _{c of} COP and OSF
And the light receiving state thereof will be described. In (a), the laser beam L is applied to the COP or OSF existing on the surface of the wafer 1.
_{When T} [P] is projected, since these shapes are close to a plane, the scattered light S _c of the [P] component is stronger than the [S] component, and the [P] component is removed by the S polarization filter 522. Since the weak [S] component is received by the PMT 523, the S / N ratio of the scattered light S _e of the foreign matter e hardly decreases. (b)
At the COP or O existing inside the wafer 1.
The SF, the transmission light T [P] is projected, they scatter scattered light S _c of the randomly polarized light into random directions, part of which enters the condenser lens 521. In this case, the condensing range of the condensing lens 521 having a converging angle of 20 ° is an angle range of 25 ° and 5 ° with respect to the surface of the wafer 1, and both angles are low angles with respect to the surface. , The angle range Δθ on the inner side corresponding to this angle range is as small as about 1 °, and the condenser lens 52
The scattered light S _c incident on 1 is only a part of the whole. Further, when the depth d of COP and OSF is large, both the transmitted light T [P] and the scattered light S _c are internally attenuated as described above. Due to these, the scattered light S _c condensed on the condenser lens 521 is greatly reduced, and the scattered light S _e of the foreign matter e S /
It hardly reduces the N ratio. From the above, the scattered light S _e of the foreign matter e is received by the PMT 523 with a good S / N ratio,
It is possible to detect the foreign matter e of 0.1 μm or less.

In the above, a laser beam of 488 nm wavelength oscillated by the argon laser tube 511 was used.
Other than this, the case of a laser beam having a wavelength of 633 nm oscillated by a helium neon laser tube will be considered. Although the refractive index of silicon for the wavelength of 633 nm is not clear for the time being, for example, when the Brewster angle θ _B is calculated by the formula (5) with respect to the refractive index of 3.43 for 3 μm described above, 73.7 ° is obtained. This angle is not much different from 76 ° with respect to 488 nm. 633 nm
Is between 488 nm and 3 μm, the Brewster angle θ _B is also considered to be between 73.7 ° and 76 °. Therefore, even if the helium neon laser tube is used as a light source and an experiment similar to the above is performed to obtain the Brewster angle θ _B with respect to the wavelength of 633 nm to configure the light projecting system 51, the same result as above can be obtained.

[0018]

FIG. 5 is a block diagram of an embodiment of a foreign matter inspection apparatus 10 equipped with the above-mentioned foreign matter detecting optical system 5. The foreign matter inspection device 10 has the foreign matter detection optics 5 shown in FIG. 3, except that a collimator lens 513 and two mirrors 514, 515 are added to the projection system 51 at the positions shown in the figure, and the light receiving system 52 is Be exactly the same. Further, the foreign matter inspection device 10 has the rotation / movable table 2 and the data processing unit 4, like the conventional foreign matter inspection device in FIG.

Hereinafter, referring to FIG.
Will be described. The wafer 1 to be inspected is placed on the rotation / movement table 2 and is rotated and moved in the radial direction. On the other hand, the P-polarized laser beam L _T having a wavelength of 488 nm from the argon laser tube 511 of the foreign matter detection optical system 5 is made parallel by the collimator lens 513, and both mirrors 514 and 515 are provided.
Then, it is focused on the spot S _P by the focusing lens 512 and projected onto the surface of the wafer 1 at a Brewster angle θ _B (up to 75 °), and the spot S _P spirally scans the surface of the wafer 1.

The randomly polarized scattered light S _e of the foreign matter e existing on the surface is condensed by the condenser lens 521 of the light receiving system 52, and the [S] component is extracted by the S polarization filter 522. On the other hand, COP and OSF existing on the surface and inside
Scatters the transmitted light T [P], and a part of the randomly polarized scattered light S _c is condensed by the condenser lens 521, and the [S] component is extracted by the S polarization filter 522. reasons, since weaker than [S] component of the scattered light S _e of the foreign object, the scattered light S _e of the foreign material e is received in PMT523 without lowering almost S / N. The foreign matter signal output from the PMT523 is input to the foreign matter detection circuit 41 of the data processing unit 4, compared with an appropriate threshold value v set therein to remove minute noise, and foreign matter e up to 0.1 μm or less,
The size of each is detected. Hereafter, as in the past,
The detected size data is input to the data editing circuit 42,
The XY coordinate values of each foreign substance e are added to edit the foreign substance data, which is printed by the printer 43 and displayed on the map.

[0021]

As described above, in the foreign matter detecting optical system of the present invention, with respect to the surface of the silicon wafer,
A major feature is that a P-polarized laser beam from a laser light source of a light projecting system is projected at a Brewster's angle θ _B , which makes the P-polarized specular reflection light from the surface have a minimum value close to zero, and further receives the light. The S-polarization filter of the system removes its influence on the scattered light of the foreign matter, and the crystal defects existing on the surface and inside and the randomly polarized scattered light of the oxide layer are partially scattered in the light receiving system. Although the light is condensed by the condenser lens, these [P] polarized components are removed by the S polarization filter and the intensity is reduced. Therefore, the scattered light of foreign matter existing on the surface is received without decreasing the S / N ratio. It is possible to detect foreign matter with a diameter of 0.1 μm or less, which was previously difficult to detect by a detector, and contributes to foreign matter inspection of silicon wafers for high integration density ICs of 64 megabits or more. Is, there is a larger.

[Brief description of drawings]

FIG. 1 is an explanatory diagram of a Brewster angle in the present invention.

FIG. 2 is a curve diagram showing experimental data such as reflectance of a silicon wafer.

FIG. 3 is a configuration diagram of a foreign matter detection optical system of the present invention.

FIG. 4 is an explanatory diagram of noise generation due to a crystal defect and an oxide layer in a foreign matter detection optical system and a light receiving state, (a)
Is the case where the crystal defects and the oxide layer are present on the surface, and (b) is the case where they are present inside.

FIG. 5 is a configuration diagram of a foreign matter inspection apparatus including a foreign matter detection optical system.

FIG. 6 is an explanatory view of a conventional foreign matter inspection device, (a)
Is a basic configuration diagram, and (b) is an explanatory diagram of light scattered by a foreign substance of the laser beam.

FIG. 7 is a cross-sectional view showing a model of crystal defects and an oxide layer existing on the surface or inside of a silicon wafer.

[Explanation of symbols]

1 ... Silicon wafer, simply wafer, 2 ... XY moving stage, 3 ... Conventional foreign matter inspection optical system, 31 ... Laser light source, 32 ...
Collimator lens, 33 ... Mirror, 34 ... Focusing lens, 35 ...
Condensing lens, 36 ... Photomultiplier tube (PMT), 4 ... Data processing unit, 41 ... Foreign substance detection circuit, 42 ... Data editing circuit, 43 ...
Printer, 5 ... Foreign matter detection optical system of the present invention, 51 ... Projection system, 511 ... Argon laser tube, 512 ... Focusing lens, 513
… Collimator lens, 514,515… Mirror, 52… Light receiving system,
521 ... Condensing lens, 522 ... S polarization filter, 523 ... Photomultiplier tube (PMT), 10 ... Foreign matter inspection apparatus equipped with foreign matter crystal optical system 5, θ ₁ ... Incident angle, θ ₂ ... Refraction angle, θ _B …
Brewster angle, n ... Refractive index, λ ... Wavelength, e ... Foreign matter,
S _e ... scattered light of foreign matter, r ... reflectance, t ... transmittance, r
[P] ... P polarized wave reflectance, r [S] ... S polarized wave reflectance, t [P] ... P polarized wave transmittance, t [S] ... S polarized wave transmittance, COP ... crystal Defect, OSF ... Oxide layer, S _c
... COP and OSF scattered light.

Claims (2)

[Claims] 1. A P-polarized laser beam is projected from a laser light source onto a surface of a silicon wafer having a refractive index n at a Brewster angle θ _B , and the P-polarized specular reflection of the projected laser beam is performed by the surface. It is provided symmetrically with the light projecting system that makes the light a minimum value close to zero, and the optical axis is set to the light receiving angle θ _R equal to the Brewster angle θ _B , and the appropriate light collecting is performed. A condensing lens having an angle and condensing randomly polarized scattered light due to a foreign substance present on the surface, an S polarizing filter for extracting an S polarized component of the condensed scattered light, and the extracted S An optical system for detecting a foreign matter on the surface of a silicon wafer, comprising: a light receiving system including a light receiver that receives a polarized component and outputs a foreign matter signal. 2. The laser light source of the projection system has a wavelength λ of 48
An argon laser tube that outputs the P-polarized laser beam of 8 nm is used, the refractive index n of the silicon wafer with respect to the wavelength λ is 4.0, and the Brewster angle θ _B with respect to the refractive index n is 76 °. Receiving angle of the light receiving system θ _R
The optical system for detecting foreign matter on the surface of a silicon wafer according to claim 1, wherein the optical angle is set to about 76 ° and the converging angle of the condenser lens is set to about 20 °.